The society was supposed to empower and strengthen the women who joined it, according to prosecutors.

But they allege Mr Raniere oversaw a "slave and master" system in his group called Nxivm, where female members were expected to have sex with him and were branded with his initials.

"Allison Mack recruited women to join what was purported to be a female mentorship group that was, in fact, created and led by Keith Raniere," Richard Donoghue, US attorney for the Eastern District in New York, said in a statement.

Image copyrightKeith Raniere Conversations/YouTubeImage caption
Keith Raniere was detained in Mexico, where he has reportedly been living for several months

Prosecutors allege that Nxivm has the features of a pyramid scheme, in which members pay "thousands of dollars" for courses to rise within its ranks.

"Mack directly or implicitly required her slaves... to engage in sexual activity with Raniere," the statement from the attorney's office claims.

"In exchange for this, Mack received financial and other benefits."

If found guilty, both Ms Mack and Mr Raniere, who has been charged with sex trafficking and is being held without bail, face at least 15 years in prison.

Masters and slaves

Allison Mack was a mainstay of the popular Superman series Smallville, which ran from 2001 to 2011.

Within Nxivm, a smaller group called DOS - or Dominus Obsequious Sororium - was created. The quasi-Latin phrase roughly translates as Master Over the Slave Women.

The group encouraged women to recruit new slaves "who in turn owed service not only to their own masters, but also to masters above them" in the pyramid scheme, prosecutors say.

Mr Raniere, 57, is alleged to have stood at the top of that pyramid, as the only man, with Ms Mack at the top level of women, immediately below him.

Investigators say Mr Raniere had a rotating group of 15 to 20 sexual partners, who were not allowed to discuss their relationship with him or have one with anyone else.

Compliance was assured in part through the use of "collateral" which had to be provided when joining - which prosecutors said included damaging information about friends or family, nude photographs, or rights to assets.

They allege that "slaves" in the group had to eat extremely low-calorie diets because of Mr Raniere's preference for thin women.

"Under the guise of female empowerment, [Ms Mack] starved women until they fit her co-defendant's sexual ideal, and she targeted vulnerable women," prosecutor Moira Kim Penza said at Friday's hearing.

In late March, Smallville co-star, Kristin Kreuk was forced to deny rumours she had acted as a recruiter for Mr Raniere.